As pointed out in said Hechler patent, the proportioning of a miscible solute in a water solvent under continuous flow in a low-cost conventional system encounters objectionable ratio variations because of variations in viscosity or in either the pressure of solvent supply, or the lift height of the solute supply the latter being either a positive or negative gauge pressure. In an endeavor to cope with these discrepancies generally referred to as ratio degradations the conventional practice, particularly with portable units, has predominantly been the use of either one of two single stage designs that are related to the relative pressure of the solute at the mixing chamber, both having their advantages and disadvantages.
In one of the designs a high vacuum, metered solute flow system is used which develops a high vacuum condition approaching zero pounds per square inch absolute (p.s.i.a.) upon the solute for metering the solute through a flow restriction at the mixing chamber level. This results in a high vacuum effect in the mixing chamber, which not only reduces the flow significance of solute lift height pressure variations on the solute supplied at the metering flow restriction but prevents ratio accuracy in the use or application of hot solutes in mixing chambers, the solutes vaporizing at the flow restriction with the pressure drop that is due to restricted flow.
Also, in the high vacuum metered solute flow system the vacuum is related to the pressure of the environment and the ratio proportioning in the first stage is adversely affected by variations in solvent pressures. Also, at higher ratios the restriction opening becomes so small as to readily become clogged and at low ratios the difficulty of maintaining a high vacuum renders this type of metering impractical.
In the other single stage design, having a low vacuum free solute flow system is provided in which the pressure acting upon the solute in the confluence chamber essentially differs only by the free-flow low vacuum lift height and vaporization or boiling point variations of the solute. Proportioning is essentially independent of variations in solvent pressure but varies with solute lift heights. Also, at higher ratios there is a greater difficulty of providing and maintaining the ratio proportioning relationship between inlet and outlet ports of the mixing chamber.
Either system can be quite accurate for certain ratios at the exact solute and solvent pressures and temperatures designed for the ratio provided their relationship then remains constant. However, only a small percentage of the commercial market would be satisfied with certain ratios and degrees of accuracy. The danger of improper solution ratios still confronts most of the users since standard concentrates are supplied and solvent pressures available significantly vary quite widely. Thus, at ratios which are available and the degradations conventionally tolerated therewith, the solute metered flow system is generally used as limited to ratios above 1 to 10 and a free-flow solute system is limited to ratios below 1 to 24.
Two-stage conventional high vacuum gap proportioners may provide a better pressure output efficiency, but solute temperatures are sometimes critical and performance is no better than single-stage proportioners with respect to ratio variations occurring in both stages.